Separation of gas mixtures



May 20, 1952 cJrscHlLLlNG 2,597,385

SEPARATIONOF GAS MIXTURES Filed Feb. 141, 194s [lfm FM Mmmm' nnnnnru/d, ,7

GASEOUS OXYG N l2 Ll UIDO YGEN COLDC-AS F |G. 'i

CLAENCE J. SCHILLING /Vl/E/VTOR A 7' TOR/VE? Patented May 20, T1952 TED DFF v 2,597,385 sE'l'ieeQN pF GAS Clarence J. Schilling, Chattanooga, Tenn., 'as- 'signor tt Pris-duets, Ihttriirateafnhhttanooga, Tenn., a r':mporation of Michigan entrati??? Febuary 11, litt ,Seriel tutti? commis. (chez-igt) the" accumulati@ 'twitch impurities" inthe vamide t' @appended drawings. in which above described;

Fig. 2 is a highly conventionalized section De: and

. ist@ Th, system is in common use and `is highly ,xygen'py means' of gas' compresso "'.thje

.expansin .Y fr@ the 'pe 'O ffl? tu, l plates Irctionat :the ai? mt@ .ga @its uitroeien `Wh` 111 .escapes at .I9 from 'the Upper sind of the 99.111131 au@ .liquid pxyein which collects ina p"ool "O in the 'base of the column. rThe compressed air within the coil 'supplies'heat .by which -a relatively large quantity .olfoXy'gen is ranorzed from 1211611001, this Vapor providing the heat by which nitrogen is evaporated out of the descending liquid air and .being constantly reconden'sed and returned Ito the pool.

tiiepted nii t' trat 2a and passes `through spin duit 2'4` and expansion' `valye`25` to reui't` 'the plates 14 it the" pri Settim* The remainder @fthe liquid r'itrgX n flows down over the lower plates `22 'tn"tthih` it' ishoht'inuoiisiy' reevjaprated. r crude'- 'ixygenr (oityger-eiriched air)v YColleeting' in pool P in the loaseI f tri lon/'er lpasses through'conduit 26 and expansion t'ggit 2,1 into ftheiitpli" ohiinh section at hiedl 'height In this upper Sstiil; substantially pireritr'ogen is selialalted escapeeat A28 while/tingen of the desired degree of purity (cpu'tfld by Column eulato) collects in a pool 'l'tli'i's point the apparatus 'and onl described tir conventional and the de'scriptic'n merely provides the environment in IWh vthe ihvjeritipiilatertg he; degcrihed is erfective and tain small but appreciable quantities of objectionable impurities. These impurities include water vapor and carbon dioxide which, being incombustible, do not present any problem so far as the avoidance of danger is concerned. They also include carbon monoxide and various hydrocarbons such as methane, ethylene, acetylene, butadiyne (biacetylene) and even heavier members of these groups, all of which form highly explosive mixtures with gaseous oxygen when in sufficient concentration. These dangerous substances may occur in minute quantity in the atmospheres of industrial districts and the acetylenes, in particular, are believed to be formed by the cracking of air cylinder lubricating oils.

Of the above substances, carbon monoxide alone has a boiling range below that of oxygen and in greater part passes out of the fractionating column with the gaseous nitrogen. The remainder are liqueiied and (with the exception of methane) are solidified in the column and tend to accumulate in the liquid oxygen fraction, in which they are suspended and perhaps to a slight extent dissolved. The following table shows the melting points of these substances, in degrees Kelvin, and also their approximate vapor pressures at 93, the temperature at which oxygen boils at 1.5 atmospheres, in the pool O of the twostage column and the single-stage column.

Melting Vapor Pressure at 93 K. Point Nitrogen Carbon monoxide Oxygen Methane Below 1 mm. Hg-: Below l mm. Hg Below l mm, Hg...

Butadiyne Carbon dioxide In the practice of the prior art, oxygen from an air fractionating column has been supplied to a liquid pump in two ways-by withdrawing liquid from below the surface of the liquid pool O, as at L in Figs. 2 and 3, or by withdrawing vapor from above the liquid level, as at V in these figures. and condensing the vapor on its way to the pump. These alternative methods produce quite different results as regards the iinal disposition of the combustible impurities.

When the liquid is withdrawn as such, the suspended impurities are eliminated from the column at the rate at which they are brought into it with the air feed, and are carried forward with the liquid oxygen into the vaporizer. If the impurities could be depended on to vaporize at the rate at which they are carried into the system, the concentration of combustibles in the gaseous oxygen could never reach a dangerous level and this alternative would be wholly satisfactory. But this continuous vaporization of the high boiling solid substances cannot be depended on, as they tend to accumulate at any point of restricted velocity in the vaporizer. Serious explosions have resulted from the sudden dislodgment of accumulations of such solids, which vaporize almost instantaneously on passage into the warm zone and lead to momentary high concentration.

The alternative of withdrawing a stream of vapor from the column and condensing it on its way to the pump is ideal from the standpoint of `retaining the combustibles within the column.

Any methane content of the air will vaporize when the methane content of the liquid oxygen in the pool becomes somewhat elevated, but the vapor pressures of the more dangerous solid acetylenes is so extremely low at column temperature that they are substantially completely retained in the pool.

The drawback to vapor withdrawal lies in a serious disturbance of normal column operation. Unless a costly external refrigerating system be provided, a column iiuid must be used to condense the vapor on its way to the pump. The only fluids having at once a sufficiently low temperature and sufficient heat absorbing capacity for this purpose are the cooled and expanded air feed to a single column (as in conduit l1 above value I8) or the liquid crude oxygen or crude nitrogen flowing from the high pressure stage to the low pressure stage of a. double column (as in conduit 26 above valve 21). The use of either of these coolants for condensing the withdrawn stream of oxygen vapor causes the evaporation of a corresponding quantity of the liquid and deprives the column of that amount of reflux, with a material lowering of fractionating eiiiciency in any given column.

Neither of these drawbacks can vbe avoided entirely without incurring the other, but the overall performance of any given oxygen plant may be improved to an important degree in the manner illustrated in Figs. 2 and 3. In this practice the lowermost plate I4 of a single column or of the low pressure section of a double column is caused to drain into an open receptacle 29 from which liquid oxygen flows through pipe 30 to the liquid pump Il of Fig. 1, this ilow being controlled by a valve 3 i`.

As the quantity of liquid ilowing from this plate is ordinarily from four to five times the quantity of oxygen contained in the air feed, and therefore of the quantity withdrawn by the pump, from one-fourth to one-fth of the flow from the plate, and a corresponding proportion of the momentary yield of combustibles, is thus passed to the vaporizer l2, the remainder overowing into the pool O.

The solids collecting in the pool are harmless so long as maintained at column operating temperature. The quantity of solids so collected is kept within bounds, Without interrupting column operation, by withdrawing relatively small amountsof liquid oxygen from the bottom of the pool, as at 32 in Fig. 2 or 33 in Fig. 3.

I claim as my invention:

1. In a liquid air fractionating column having a series of bubble plates ,each having a downcomer, and a reservoir for the reception of liquid oxygen: a receptacle arranged below the final plate of said series tol seal the downcomer from said plate, said receptacle having an opening for the overow of excess liquid to said reservoir, and a conduit arranged to conduct liquid from said receptacle to the exterior of said column.

2. In a liquid air fractionating column having a series of fractionating plates and a reservoir for the reception of liquid oxygen, the combination comprising downcomer means for the lowermost fractionating plate, a receptacle arranged below the lowermost fractionating plate into which/'the downcomer means projects, an opening in the receptacle arranged to maintain a liquid level therein to seal the downcomer means and to permit excess liquid to overow into the reservoir, a conduit arranged to conduct liquid from the receptacle to the exterior of the column. and means controlling the conduitto causeleSS liquid to be conducted from the receptacle than enters the receptacle through the downcomer means.

3. In a liquid air fractionating column having a series of fractionating plates and a reservoir for the reception of liquid oxygen, the combination comprising a receptacle arranged below the lowermost fractionating plate, means for directing a stream of liquid oxygen from the lowermost fractionating plate into said receptacle, means for withdrawing a stream of liquid oxygen from the lowermost portion of the receptacle to the exterior of the column, and control means associated with the last claimed means whereby the last claimed stream is less than the rst claimed stream and substantially constantly related to the quantity of oxygen separatedfrom the air fed to the column. l

CLARENCE J. SCHILLING.

REFERENCES CITED The following references are of record in the file of this patent: 

1. IN A LIQUID AIR FRACTIONATING COLUMN HAVING A SERIES OF BUBBLE PLATES EACH HAVING A DOWNCOMER, AND A RESERVOIR FOR THE RECEPTION OF LIQUID OXYGEN: A RECEPTACLE ARRANGED BELOW THE FINAL PLATE OF SAID SERIES TO SEAL THE DOWNCOMER FROM SAID PLATE, SAID RECEPTACLE HAVING AN OPENING FOR THE OVERFLOW OF EXCESS LIQUID TO SAID RESERVOIR, AND A CONDUIT ARRANGED TO CONDUCT LIQUID FROM SAID RECEPTACLE TO THE EXTERIOR OF SAID COLUMN. 